1. Field of the Invention
The present invention relates to a motor and an electronic apparatus using the same.
2. Description of the Related Art
In electronic apparatuses such as laser printers, a paper feed roller (driven member) provided in a main body case is coupled via a deceleration mechanism to a driving shaft of a motor. When this motor is driven, the paper feed roller rotates and feeds paper to a predetermined portion.
As this motor, a brushless DC motor that ordinarily is used includes: a stator on whose outer circumference a plurality of magnetic poles are arranged at a first predetermined interval; and a rotor that is rotatably disposed around the stator. An inner circumferential face of the rotor is provided with a magnet magnetized to have opposite polarities at a second predetermined interval.
In this sort of motor, ordinarily, in order to arrange the magnet of the rotor as close as possible to a magnetism-detecting element that magnetically detects rotation of the rotor, the size of the magnet in a direction parallel to a motor-driving shaft is set larger than the size of a magnetic pole base of the stator in the same direction. In this case, an extended portion that extends in a direction substantially parallel to the magnet often is formed on both sides of a magnetic pole base, at outer circumferential ends of the magnetic poles of the stator (see JP H9-285044A and JP 2007-244004A, for example). Accordingly, the area in which the magnet of the rotor and the magnetic poles of the stator oppose each other increases, and, thus, the driving force and the driving efficiency of the motor can be increased.
The extended portion has an effect of causing magnetic fluxes from the magnet to flow thereinto, and, thus, more magnetic fluxes from the magnet can be directed to the magnetic poles of the stator in the case where the extended portion is provided than in the case where no extended portion is provided. Accordingly it is considered that the driving force and the driving efficiency can be increased by forming an extended portion at outer circumferential ends of the magnetic poles of the stator.
However, according to investigations of the present inventors, the driving force cannot be necessarily increased simply by providing an extended portion.
The extended portion ordinarily is formed by bending plate-shaped members constituting the stator so as to be substantially parallel to the magnet. In the case where, in a plate-shaped member that is the closest to the magnet among the plate-shaped members constituting the extended portion, the diameter (hereinafter; referred to as an “outer diameter”) of a face (outer face) of the bent portion opposing the magnet is large, the distance between the bent portion and the magnet increases, and, thus, the effect of causing magnetic fluxes from the magnet to flow into the bent portion is lowered. As a result, the driving force and the driving efficiency cannot be improved as expected.
Furthermore, magnetic fluxes from the magnet flowing into the extended portion pass through the bent portion of the plate-shaped member. However; due to a processing strain that occurs during the bending processing of the plate-shaped member; a magnetic properties-deteriorated region is formed in the bent portion. In this magnetic properties-deteriorated region, magnetic saturation easily occurs. When magnetic saturation occurs, iron loss increases. As a result, the driving force and the driving efficiency cannot be improved.